LSM 12 was originally built as an Ericsson (open) cycle engine. These have the advantage of not requiring a cold end heat exchanger (which is more critical then the hot-end heat exchanger in Stirling engines by a factor of about 3/1), but the disadvantage of requiring valves- with their attendant friction, flow, and sealing losses.

During the 19th century, open cycle air engines were championed by Swedish engineer John Ericsson. In 1853 he developaed a four cylinder, 200KW, 4.3m bore, 1.8m stroke version for a 2,200 ton ship. Named "The Ericsson" which was not commercially successful. There has been little or no development of open cycle engines since then. Ericsson went on to design closed (Stirling cycle) engines such as the popular Ericsson Pumping Engine range and made many other notable contributions to engineering.

Andreas Load Testing LSM 12

In open cycle form, LSM 12 drew in a fresh charge of cold air every cycle, exhausting it after heating and expansion, but even after a year of trialing and many modifications, LSM12 in this form never once ran, not even nearly. The problem seems to have been inadequate sealing in the valve mechanism rather than mechanical and flow losses- and this is the area I will focus on in any future attempts at an open cycle air engine.

In early 2011, LSM 12 was converted to closed (Stirling) cycle form- in which a quantity of air is alternately heated and cooled.

This required extending the displacer by 130 mm, applying crank control to the displacer (phased to lead the piston by 90degrees), and adding a cold end heat exchanger.

LSM 12 in Piwakawaka

LSM 12 then started and ran immediately, so enthusiastically that a Watts type governor that releases internal pressure to prevent over-speeding (threatening damage) was fitted.

In November '11, LSM-12 was installed in Piwakawaka, which it powered very successfully until replaced by the more refined and compact LSM14 engine in March '12.